This invention relates to power amplifiers and, more specifically, to a circuit and method for controlling the bias of a hetero-junction bipolar transistor (HBT) power amplifier.
Hetero-junction bipolar transistor (HBT) power amplifiers are becoming the standard for cellular applications due to their high power density and reduction in die size. Unfortunately, biasing these transistors with a constant current poses some difficulty. The voltage supply limitation typical to mobile applications combined with a relatively high Vbe of HBT devices make traditional integrated methods unusable.
FIG. 1 shows a typical diode based biasing control of an HBT transistor. A power amplifier Qpa HBT 100 is biased by a diode configured transistor 110 where the base and collector are shorted together and receive a current through a resistor 120 and supply voltage Vref 130. This configuration requires that a separate voltage Vref 130 (different from the battery voltage Vbat 140 supplied to the collector of the power amplifier 110) be applied to the diode transistor and the biased base of the power amplifier in order to tightly control the biasing current. This configuration leads to several problems for power amplifier applications in mobile communications. Typically, the power amplifier 100 is N times larger than the diode transistor 110 leading to current stealing. Additionally, Rref 120 needs to be large to provide stability over variations in temperature and process, but needs to be small to provide enough current to properly bias the power amplifier, resulting in a circuit that would require a stable reference which supplies a prohibitively large amount of current and is not a viable circuit for power amplifiers in mobile communications applications.
Another solution, shown in FIG. 2, solves the problem of current stealing by using a current mirror with an emitter follower to bias the current supplied to the power amplifier""s base. The base of a power amplifier transistor 200 is connected to a base of mirrored transistor 210 and the emitter of a emitter follower transistor 250. The collector of the mirrored transistor 210 is connected to the base of the emitter follower transistor 250 and is connected to a reference voltage 230 through a reference resistor 220 while the collector of the emitter follower transistor 250 is connected to the battery voltage 240 which is also connected to the collector of the power amplifier transistor 200 through some impedance 270. However, this type of circuit is not viable because gallium arsenide (GAS) HBT power amplifiers as now used have Vbe""s in the order of 1.4 volts while battery voltage supplies are required to be in the range of 2.7 volts. To control the voltage at the base of the power amplifier, the voltage supply, Vref 230, would need to be greater than is desirable for mobile communication applications and the solution is therefore not viable.
What is needed is a method of controlling the bias of an HBT power amplifier transistor used in RF applications that provides stability over temperature and process.